Chapter 20
Properties of solutions
Making solutions
 What
the solute and the solvent are
– Whether a substance will dissolve.
– How much will dissolve.
 A substance dissolves faster if It is stirred or shaken.
 The particles are made smaller.
 The temperature is increased.
 Why?
Making solutions
 In
order to dissolve the solvent
molecules must come in contact with
the solute.
 Stirring moves fresh solvent next to
the solute.
 The solvent touches the surface of
the solute.
 Smaller pieces increase the amount
of surface of the solute.
Temperature and Solutions
 Higher
temperature makes the
molecules of the solvent move
around faster and contact the solute
harder and more often.
 Speeds up dissolving.
 Usually increases the amount that
will dissolve.
How Much?
 Solubility-
The maximum amount of
substance that will dissolve at that
temperature (usually g/L).
 Saturated solution- Contains the
maximum amount of solid dissolved.
 Unsaturated solution- Can dissolve more
solvent.
 Supersaturated- A solution that is
temporarily holding more than it can, a
seed crystal will make it come out
Liquids
 Miscible
means the that to liquids
can dissolve in each other.
 Immiscible means they can’t
What affects solubility?
 For
solids in liquids as the
temperature goes up the solubility
goes up.
 For gases in a liquid as the
temperature goes up the solubility
goes down.
 For gases in a liquid- as the pressure
goes up the solubility goes up.
Measuring Solutions
Concentration
 A measure
of the amount of solute
dissolved in a certain amount of solvent.
 Concentrated solution has a large amount
of solute.
 Dilute solution has a small amount of
solute
 Sometimes g/l or g/mL of g/100 mL.
 But chemical reactions don’t happen in
grams
Molarity
 The
number of moles of solute in 1
Liter of the solution.
 M = moles/Liter
 What is the molarity of a solution
with 2.0 moles of NaCl in 4.0 Liters
of solution.
 What is the molarity of a solution
with 3.0 moles dissolved in 250 mL
of solution.
Making solutions
 Pour
in a small amount of solvent
 Then add the solute and dissolve it
 Then fill to final volume.
 M x L = moles
 How many moles of NaCl are needed to
make 6.0 L of a 0.75 M NaCl solution?
 How many grams of CaCl2 are needed
to make 625 mL of a 2.0 M solution?
Making solutions
 10.3
g of NaCl are dissolved in a
small amount of water then diluted to
250 mL. What is the concentration?
 How many grams of sugar are
needed to make 125 mL of a 0.50 M
C6H12O6 solution?
Dilution
Adding water to a solution
 The
Dilution
number of moles of solute doesn’t
change if you add more solvent.
 The moles before = the moles after
 M1
x V1 = M2 x V2
 M1 and V1 are the starting concentration
and volume.
 M2 and V2 are the starting concentration
and volume.
 Stock solutions are pre-made to known M
Practice
 2.0
L of a 0.88 M solution are diluted
to 3.8 L. What is the new molarity?
 You have 150 mL of 6.0 M HCl. What
volume of 1.3 M HCl can you make?
 Need 450 mL of 0.15 M NaOH. All
you have available is a 2.0 M stock
solution of NaOH. How do you make
the required solution?
Percent solutions
 Percent
means per 100 so
 Percent by volume
= Volume of solute x 100%
Volume of solution
 indicated %(v/v)
 What is the percent solution if 25 mL
of CH3OH is diluted to 150 mL with
water?
 Percent
Percent solutions
by mass
= Mass of solute(g)
x 100%
Volume of solution(mL)
 Indicated %(m/v)
 More common
 4.8 g of NaCl are dissolved in 82 mL of
solution. What is the percent of the
solution?
 How many grams of salt are there in 52
mL of a 6.3 % solution?
Colligative Properties
Depend only on the number of
dissolved particles
Not an what kind of particle
Vapor Pressure
 The
bonds between molecules keep
molecules from escaping.
 In a solution, some of the solvent is
busy keeping the solute dissolved.
 Lowers the vapor pressure.
 Electrolytes form ions when
dissolved - more pieces.
 NaCl  Na+ + Cl- 2 pieces
 More pieces bigger effect.
Boiling Point Elevation
 The
vapor pressure determines the
boiling point.
 Lower vapor pressure - higher
boiling point.
 Salt water boils above 100ºC
 The solvent determines how much.
Freezing Point Depression
 Solids
form when molecules make
an orderly pattern.
 The solute molecules break up the
orderly pattern.
 Makes the freezing point lower.
 Salt water freezes below 0ºC
 How much depends on the solvent.
Molality
a
new unit for concentration
 m = Moles of solute
kilogram of solvent
 m = Moles of solute
1000 g of solvent
 What is the molality of a solution with
9.3 mole of NaCl in 450 g of water?
Why molality?
 The
size of the change in boiling
point is determined by the molality.
 DTb = Kb x m x n
 DTb is the change in the boiling point
 Kb is a constant determined by the
solvent(pg 387).
 m is the molality of the solution.
 n is the number of pieces it falls into
when it dissolves.
What about Freezing?
 The
size of the change in freezing
point is determined by the molality.
 DTf = -Kf x m x n
 DTf is the change in the boiling point
 Kf is a constant determined by the
solvent(pg 388).
 m is the molality of the solution.
 n is the number of pieces it falls into
when it dissolves.
Problems
 What
is the boiling point of a solution
made by dissolving 1.20 moles of
NaCl in 750 g of water?
 What is the freezing point?
 What is the boiling point of a solution
made by dissolving 1.20 moles of
CaCl2 in 750 g of water?
 What is the freezing point?